1. Field of the Invention
The present invention relates to free radical radiation curable liquids for de-inking substrates, more particularly for de-inking substrates printed with free radical radiation curable inkjet inks.
2. Description of the Related Art
Sustainable development is a pattern of economic growth in which resource use aims to meet human needs while preserving the environment so that these needs can be met not only in the present, but also for generations to come. In the paper printing industry, a good recyclability of printed products is a crucial feature for the sustainability of the graphic paper loop.
Radiation curable inks allow printing on a wide range of substrates, including substantially non-absorbing substrates, such as e.g. glass and plastic packaging materials. The recyclability and re-use by de-inking substrates printed with radiation curable inks has proven to be more difficult than when printed with aqueous or solvent based inks.
Curing of radiation curable monomers and oligomers on paper and textile substrates results in a polymeric network entangled with the paper or textile fibres, making it difficult to separate the fibres from the polymer network in a paper recycling process.
In other cases, easily removable temporary inkjet prints are required on permanent substrates or objects. For example on a trade fair, advertisements or directions to a certain exhibition booth are printed on glass or concrete walls and floors and should afterwards be easily de-inked and re-printed with advertisements and directions for the next trade fair.
De-inking processes of aqueous or solvent based ink printed matter has been disclosed extensively. For example, U.S. Pat. No. 4,162,186 (DESOTO) discloses a de-inking process for removing ink and other substances from printed paper by using an aqueous de-inking solution containing a mixture of certain surfactants. Other methods for paper and textile recycling are disclosed in EP 927277 A (RHONE POULENC), U.S. Pat. No. 5,314,580 (MAUVIN), U.S. Pat. No. 5,401,360 (CENTRE TECHNIQUE) and EP 2447414 A (OFFICE2OFFICE).
In general, UV inks present difficulties during the de-inking of the printed paper by means of the conventional techniques, due to the excessive resistance to hydrolysis of the cross-linked film of ink.
U.S. Pat. No. 4,035,320 (UNION CAMP) discloses radiation curable, solventless printing inks including a reaction product of an equivalent excess of an acrylic type acid with a particular class of polyester polyol. Although a caustic treatment can be used for de-inking, the viscosity of these printing inks are around 10 Pa·s or more due to the polyacrylate reaction product, making them unsuitable for inkjet printing applications.
U.S. Pat. No. 4,070,259 (UCB) discloses radiation curable inks consisting essentially of Diels-Alder reaction products. The de-inking of paper printed with such UV inks can be improved by the choice of appropriate dienophiles, such as those containing phosphite, phosphate or amine radicals in their molecules or those derived from polyether- or polyester-polyols. The requirement that the radiation curable ink should consist essentially of Diels-Alder reaction products, limits the options for adjusting ink properties such as flexibility, abrasion resistance, viscosity, etc.
U.S. Pat. No. 7,951,267 (NOVOZYMES) discloses the use of certain lipolytic enzymes such as cutinases and lipases for de-inking purposes. The enzymatic treatment requires a neutral pH between about 6 and 8 for preserving the integrity of the enzymes. However, it would be advantageous to be able to use the common caustic treatment as in place for recycling paper from aqueous or solvent based ink printed matter and to avoid the use of expensive enzymes.
Radiation curable inks are also often used for outdoor applications, therefore any hydrolysis of radiation cured printed matter should preferably not occur at a neutral or acid pH, i.e. rain and acid rain.